Electric power systems comprise subsystems that are used to transform and/or condition electrical power in a variety of applications. For example, DC/AC (DC to AC) power inverter systems are used to invert DC voltage/power to a suitable AC voltage/power, such as standard 50/60 Hz 380V/480V AC. DC/AC inverters are widely used in a variety of power implementations. For instance, power inverter systems may transform power from a standalone power source (such as an array of photovoltaic cells, fuel cell system, micro-turbine, or flywheel, and the like) for use in a standalone application and/or for export to, or by, a power grid.
Balanced (or symmetrical) phase voltages of a 3-phase power inverter comprise identical magnitude for each phase and a symmetrical 120° phase shift between any two phases under all load conditions. When a 3-phase DC/AC inverter is used as a power source, keeping the DC/AC inverter's three output phase voltages balanced is desirable because certain types of 3-phase loads (such as 3-phase motors, 3-phase transformers, and other types of loads) operate more efficiently with balanced input voltages.
In some applications, the 3-phase DC/AC inverter supplies an unbalanced 3-phase load. In such unbalanced load conditions, balanced output voltages from the DC/AC inverter may become unbalanced, because there is an internal impedance in the DC/AC inverter. 3-phase 3-wire DC/AC inverters have only two independent current paths, which make only two independently controllable phase voltages available. Thus, sufficient phase voltage control for a 3-phase 3-wire DC/AC inverter under unbalanced conditions is difficult. 3-phase 4-wire DC/AC inverters have three phase lines plus a neutral line that form independent current paths for each phase. A 3-phase 4-wire DC/AC inverter can thus be treated as three single-phase inverters. Because this topology is useful for balance control of the phase voltages, 3-phase 4-wire DC/AC inverters are widely used in unbalanced load applications.
Using a 3-phase 4-wire DC/AC inverter as a voltage source in unbalanced operations raises control issues with respect to achieving independent phase voltage control for the DC/AC inverter's 3-phase output voltages. Independent phase voltage control is advantageous so that the DC/AC inverter can deliver any required voltage for each phase to meet different applications. Delivering three balanced voltages at unbalanced load conditions is a common application. As mentioned above for unbalanced load conditions, balanced output voltages from the DC/AC inverter may become unbalanced due at least in part to the internal impedance in DC/AC inverters. Moreover, 3-phase load equipment (such as a 3-phase AC motor, 3-phase transformer, etc.) may have trouble operating with a voltage source that provides unbalanced phase voltages. For instance, unbalanced phase output voltages from a voltage source may cause a 3-phase motor to generate torque ripple or vibration. Keeping phase output voltages balanced during unbalanced operating conditions is a useful performance feature for a 3-phase 4-wire DC/AC inverter, but can be difficult to obtain.
Delivering three unbalanced phase voltages under balanced or unbalanced load conditions is another application of a 3-phase 4-wire DC/AC inverter, for example, for use as special power supplies as test equipment. 3-phase 4-wire DC/AC inverters are further used to handle severe unbalanced load operations where some load is applied in single phase, some load in two phases, and some load in three phases.
To obtain balanced output voltages for a DC/AC inverter under severe unbalanced load conditions and/or to provide unbalanced output voltages from a DC/AC inverter that are to be supplied to balanced or unbalanced loads, reliable independent phase voltage control is needed but is not adequately addressed by existing schemes.